To satisfy demands for wireless data traffic having increased since commercialization of 4th-Generation (4G) communication systems, efforts have been made to develop improved 5th-Generation (5G) communication systems or pre-5G communication systems. For this reason, the 5G communication system or the pre-5G communication system is also called a beyond-4G-network communication system or a post-Long-Term Evolution (LTE) system.
To achieve a high data rate, implementation of the 5G communication system in an ultra-high frequency (mmWave) band (e.g., a 60 GHz band) is under consideration. In the 5G communication system, beamforming, massive multi-input multi-output (MIMO), full dimensional MIMO (FD-MIMO), an array antenna, analog beamforming, and large-scale antenna technologies have been discussed to alleviate a propagation path loss and to increase a propagation distance in the ultra-high frequency band.
For system network improvement, in the 5G communication system, techniques such as an evolved small cell, an advanced small cell, a cloud radio access network (RAN), an ultra-dense network, a device to device (D2D) communication, a wireless backhaul, a moving network, cooperative communication, coordinated multi-points (CoMPs), and interference cancellation have been developed.
In the 5G system, advanced coding modulation (ACM) schemes including hybrid frequency-shift keying (FSK) and quadrature amplitude modulation (QAM) modulation (FQAM) and sliding window superposition coding (SWSC), and advanced access schemes including filter bank multi carrier (FBMC), non-orthogonal multiple access (NOMA), and sparse code multiple access (SCMA) have been developed.
Recently, several broadband wireless technologies have been developed to meet the growing number of broadband subscribers and to provide more and better applications and services. Examples of broadband systems include Code Division Multiple Access 2000 (CDMA 2000), 1× Evolution Data Optimized (1×EVDO) and Ultra Mobile Broadband (UMB) systems of the 3rd-Generation Partnership Project 2 (3GPP2), Wideband Code Division Multiple Access (WCDMA), High Speed Packet Access (HSPA) and Long Term Evolution (LTE) systems of the 3rd Generation Partnership Project (3GPP), and Mobile Worldwide Interoperability for Microwave Access (WiMAX) systems of the Institute of Electrical and Electronics Engineers.
With the advent of new schemes and devices, there is an increasing need for a mobile communication system having large capacity, coverage and better reliability to support critical communication applications, real time applications, and web applications. One of ways to meet the need is allowing a user equipment (UE) to simultaneously connect to multiple enhanced NodeBs (eNBs) or base stations (BSs).
In a current state-of-the-art mobile communication system based on the 3GPP LTE standard, a UE is allowed to connect to two eNBs. One of the eNBs is a master eNB (MeNB) and another one of the eNBs is a secondary eNB (SeNB). The UE may receive or transmit data from or to these eNBs at the same time. In the current approach, when the signal quality of the SeNB with which the UE is communicating is degraded, the MeNB triggers release of the existing SeNB to perform an operation of releasing the existing SeNB and adding a new SeNB.